A solid oxide fuel cell (SOFC) is an electrochemical device for the generation of electrical energy through the electrochemical oxidation of a fuel gas (usually hydrogen-containing). The device typically uses an oxygen-ion conducting metal-oxide derived ceramic as its electrolyte. Single fuel cells are connected via interconnects to form fuel cell stacks. The interconnect provides gas flow paths to and from the cell, and carries electrical current away from the cells.
An effective interconnect should be gas impermeable, to prevent mixing of the oxidant on one side of the interconnect with fuel on the other side of the interconnect; have high electrical conductivity, to allow transfer of the electric current away from the cell, with a low contact resistance at the interconnect/electrode interface. Further, a high thermal conductivity is desirable to allow the transfer of heat away from the individual cells, and to evenly distribute the heat loading within the stack of fuel cells thereby reducing thermal stresses associated with changes in temperature in a fuel cell layer and within the stack of fuel cells. In addition, the interconnect should have a similar thermal expansion co-efficient to the cell components, to minimise mechanical stress during cycling. The interconnect should also be stable to the conditions found in the stack, for instance by having good chemical stability relative to the fuel and oxidant, and good mechanical stability at operation temperatures. Further, the interconnect and the metal supported fuel cell substrate should have well matched thermal expansion characteristics over the operating temperature range during operation of the fuel cell. The interconnect should also allow for simple methods of joining to the metal supported fuel cell substrate, to enable a gas tight seal to be formed and to allow for efficient current transfer and a robust join over the life of the metal supported fuel cell and stack. This joining is simply done by welding the interconnect to the metal substrate, such as laser welding the interconnect to the fuel side of the metal supported substrate.
SOFC's typically operate at temperatures in the range 700-900° C., however, such high temperature operation results in long start up times, and the need to use specialist materials that are robust to long term exposure to high temperatures. SOFC's that can operate at lower temperatures (for instance, less than 650° C.) have been developed by the applicant as exemplified by their patent number GB 2,368,450 which describes a metal-supported SOFC.
However, a problem associated with low temperature SOFC's is the slow formation of a passivating chromium oxide scale on the metal components (for instance, on the stainless steel substrates and interconnects). The scale forms a protective layer on the steel, preventing corrosion. At temperatures below 650° C., the rate of chromium diffusion from a steel to its surface is low. In addition, where the steel surface is exposed to flowing humidified air (as is often the case) such as on the oxidant side of the interconnect during operation of the fuel cell, the slow formation of the chromium oxide scale may result in it evaporating faster than it is formed, leaving the steel unprotected. Further, under the operating environment of a metal supported SOFC interconnect, the corrosion of the steel may be accelerated on the oxidant side (the side exposed to air), as hydrogen may diffuse through the steel from the fuel side of the interconnect. This promotes the formation of iron oxides on the oxidant side of the steel causing corrosion of the interconnect steel instead of passivation.
In view of this, it has been proposed to protect the interconnects in a low temperature SOFC stack from corrosion by providing an interconnect plate which is made of ferritic stainless steel, that is coated on the oxidant side, the coating preventing chromium evaporation from the surface. However, whilst this method has the benefit that contact resistance remains acceptably low, the formation of the chromia layer remains unpredictable, and so corrosion of the steel, particularly in the interconnect region can still occur. The invention is intended to overcome or ameliorate at least some aspects of this problem.